1. Field of the Invention
The present invention relates to a method of forming a select line in a NAND type flash memory device, and more particularly, to a method of forming a select line in a NAND type flash memory device capable of preventing degradation in electrical characteristics due to voltage drop when the bias is applied to a polysilicon layer for a floating gate having a high resistance, in the select transistors having the same structure to the flash memory cells.
2. Background of the Related Art
A NAND type flash memory of a stack structure operates in a block unit. At this time, the blocks are divided and selected, using the select transistor.
FIG. 1 is a layout view of a conventional NAND type flash memory array, FIG. 2 is a cross-sectional view of the memory array taken along lines A-Axe2x80x2 in FIG. 1, and FIG. 3 is a cross-sectional view of the memory array taken along lines B-Bxe2x80x2 in FIG. 1.
Referring to FIG. 1, FIG. 2 and FIG. 3, device isolation films 102 are formed at the semiconductor substrate 101 to define a plurality of active regions ACT that are parallel one another. Further, drain select lines DSL0, DSL1, . . . , firstxcx9cnth word lines WL1xcx9cWLn and source select lines SSL1, SSL2, . . . are formed vertically to the active region ACT on the semiconductor substrate 101. The lines have a stack structure of a tunnel oxide film 103, a floating gate 104, a dielectric film 105 and a control gate 106. The control gate 106 has a stack structure of a polysilicon layer and a silicide layer. Meanwhile, impurity regions 107d, 107 and 107c are formed at the active region ACT between the drain select lines DSL0, DSL1, . . . , the firstxcx9cnth word lines WL1xcx9cWLn and the source select lines SSL1, SSL2, . . . . At this time, the impurity region 107d formed at the edge of the drain select line DSL1 on the opposite side of the first word line WL1 serves as the drain. The impurity region 107c formed at the edge of the source select line SSL1 on the opposite side of the nth word line WLn functions as the source.
Meanwhile, a drain select transistor Td is formed at a region where the drain select line DSL1 and the active regions ACT intersect. A ground select transistor Ts is formed at a region where the source select line SSL1 and the active regions ACT intersect. Likewise, flash memory cells Cell1xcx9cCelln are formed at a region where the firstxcx9cnth word lines WL1xcx9cWLn and the active regions ACT intersect. At this time, the active region ACT is isolated by the isolation film 102. However, in order to easily connect the active regions ACT between the ground select transistors Ts using the ground terminals, the active regions ACT may be formed so that the active regions ACT between the ground select transistors Ts may be connected (it is shown in the drawing that they are isolated one another). Thereby, a block B100 having the drain select transistor Td, a plurality of flash memory cells Cell1xcx9cCelln and a ground select transistor Ts, all of which are serially connected to the active region ACT, is formed on the semiconductor substrate 101.
In the above, the floating gates 103 of the flash memory cells Cell1xcx9cCelln are isolated one another on the device isolation region by means of the floating gate isolation FGI pattern. However, the drain select lines DSL0, DSL1, . . . and the ground select lines SSL1, SSL2, . . . are not isolated even on the device isolation film 102 but all the layers 102xcx9c106 are consecutively connected, as shown in FIG. 3.
At this time, as the drain select transistor Td and the source select transistor Ts are formed to have the structure of the flash memory cell, it is required that a high voltage be applied to the select line DSL or SSL or the floating gate in order to obtain the normal transistor operation, or the dielectric films 105 between the control gate 106 and the floating gate 104 in the drain select lines DSL0, DSL1, . . . and the source select lines SSL1, SSL2, . . . be removed.
In this case, abrupt voltage drop occurs due to high resistance of the floating gate. Further, as the degree of integration in the device becomes higher, it is difficult to remove the process margin for removing the dielectric films 105 between the control gate 106 and the floating gate 104 in the drain select lines DSL0, DSL1, . . . and the source select lines SSL1, SSL2, . . . . Due to this, there is a disadvantage that the degree of difficulty in the entire processes becomes higher.
Accordingly, the present invention is contrived to substantially obviate one or more problems due to limitations and disadvantages of the related art, and an object of the present invention is to provide a method of forming a select line in a NAND type flash memory device capable of improving electrical characteristics by minimizing generation of voltage drop, and simplifying the process steps by obviating the process of removing the dielectric film for electrically connecting the floating gate and the control gate.
In order to accomplish the above object, in the select line having the stack structure of the floating gate, the dielectric film and the control gate, the control gate is patterned so that a first projection is formed at the edge of the control gate and the floating gate is formed by means of the self-aligned etch process. At this time, the floating gate is patterned so that a second projection the one end of which overlaps the first projection is formed at the edge of the floating gate. The first and second projections are then electrically connected using the contact plugs and the metal line, whereby a voltage is simultaneously applied to the control gate of a low resistance and the floating gate of a high resistance.
In a preferred embodiment, a method of forming the select line of NAND type flash memory devices according to the present invention is characterized in that it comprises the steps of sequentially forming a tunnel oxide film and a first polysilicon layer on a semiconductor substrate in which an isolation film is formed, and then performing a first patterning process in the direction of bit lines, forming a dielectric film, a second polysilicon layer and a silicide layer on the entire structure, performing a second patterning process for the silicide layer and the second polysilicon layer so that the first projection is formed at the edge of the silicide layer and the second polysilicon layer on the device isolation region between drain select line regions, performing a third patterning process for the dielectric film and the first polysilicon layer to form drain select lines so that a second projection the one end of which overlaps the first projection is formed at the edge of the first polysilicon layer on the device isolation region between the drain select lines, forming an interlayer insulating film on the entire structure and then forming a contact hole through which the first and second projections are opened, and burying the contact hole with a conductive material to form contact plugs and also forming a metal line connecting the contact plugs formed on the first and second projections on the interlayer insulating film.
In the above, during the second patterning process, in the cell region, the silicide layer and the second polysilicon layer are patterned in order to form word lines.
The third patterning process is performed with the etch mask formed on the second projection so that the second projection is formed at the edge of the first polysilicon layer. During the third patterning process, in the cell region, a self-aligned etch process is performed in order to form the word lines, so that the dielectric film and the first polysilicon layer are patterned.
The method further comprises the step of forming insulating film spacers at the sidewalls of the drain select lines, before the interlayer insulating film is formed after the third patterning process.
Upon formation of the contact hole and the contact plug, the contact hole and the contact plugs for connecting the bit line and the active region are formed even at the active region of the cell region.
In another preferred embodiment, a method of forming a select line of NAND type flash memory devices according to the present invention is characterized in that it comprises the steps of sequentially forming a tunnel oxide film and a first polysilicon layer on a semiconductor substrate in which an isolation film is formed, and then performing a first patterning process in the direction of bit lines, forming a dielectric film, a second polysilicon layer and a silicide layer on the entire structure, performing a second patterning process for the silicide layer and the second polysilicon layer by means of an etch process using a control gate mask, forming an interlayer insulating film on the entire structure and then forming a contact hole through which a given portion of the first and second polysilicon layer is opened, and burying the contact hole with a conductive material to form contact plugs and also forming a metal line connecting the contact plugs formed on the first and second projections on the interlayer insulating film.
In the above, during the second patterning process, in the cell region, the silicide layer and the second polysilicon layer are patterned in order to form word lines.
The method further comprises the step of performing a third patterning process for the dielectric film and the first polysilicon layer in the cell region by means of the self-aligned etch process, before the interlayer insulating film is formed after the second patterning process. The method further comprises the step of forming insulating film spacers at the sidewalls of the source select line, before the interlayer insulating film: is formed after the second patterning process.
Upon formation of the contact hole and the contact plug, the contact hole and the contact plugs for connecting the bit line and the active region are formed even at the active region of the cell region.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In another aspect of the present invention, it is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.